The spinal column of humans provides support to the body and protection to the delicate spinal cord and nerves. The spinal column comprises a series of vertebrae stacked on top of each other. Each vertebra has a relatively large vertebral body that is located in the anterior portion of the spine and provides the majority of the weight bearing support of the vertebral column. Each vertebral body has relatively strong bone comprising the outside surface of the body and weak bone comprising the center of the body. Situated between each vertebral body is an intervertebral disc that provides for cushioning and dampening of compressive forces to the sinal column. Located just posterior to the vertebral body and intervertebral disc is the vertebral canal containing the delicate sinal cord and nerves. Posterior to the spinal canal are the different articulating processes of the vertebra.
Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in the lumbar or cervical spine) and other disorders, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients who suffer from such conditions usually experience extreme and debilitating pain and often neurologic deficit in nerve function.
A technique known as spinal fixation uses surgical implants which mechanically immobilize areas of the spine assisting in the eventual fusion of the treated adjacent vertebrae. Such techniques have been used effectively to treat the above described conditions and, in most cases, to relieve pain suffered by the patient. However, there are some disadvantages to the present fixation devices.
One technique for spinal fixation includes the immobilization of the spine by the use of spine rods that run generally parallel to the spine. In practicing this technique, the posterior surface of the spine is isolated and bone screws are first fastened to the pedicles of the appropriate vertebrae or to the sacrum and act as anchor points for the spine rods. The bone screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process. Clamp assemblies join the spine rods to the screws. The spine rods are generally bent to achieve the desired curvature of the spinal column. These types of systems are very stable but require implanting screws into each vertebra over the area to be treated. Also, since the pedicles of vertebrae above the second lumbar vertebra (L2) are very small, only small bone screws can be used which sometimes do not give the needed support to stabilize the spine. To stabilize the implanted system sufficiently, one vertebra above and one vertebra below the area to be treated are often used for implanting pedicle screws. The rods and clamps are surgically fixed to the spine from a posterior approach.
Anterior fixation devices have also been used such as anterior plate systems. One type of anterior plate system involves a titanium plate with unicortical titanium bone screws that lock to the plate and are placed over the anterior surface of a vertebral body. Another type of anterior plate system used less frequently nowadays involves the use of bicortical screws that do not lock to the plate. The bone screws have to be long enough to bite into both sides of the vertebral body (cortex) to gain enough strength to obtain the needed stability. These devices are difficult to place due to the length of the screws and damage occurs when the screws are misplaced.
A third type of anterior fixation device comprises a hollow cylinder, usually a hollow cylindrical titanium cage, that is externally threaded. The externally threaded cage is screwed into place between to adjacent vertebrae. Bone grafts from cadavers or the pelvis are then packed into the hollow center of the device. Bone morphogenic protein (which is not yet commercially available) or other substances that promote bone growth can also be placed in the hollow center of the device. The cage is porous such that bone can grow through the device and fuse the two adjacent vertebrae. The are many disadvantages to this device. First, it is very difficult to align. Second, it requires drilling a large hole between two adjacent vertebral bodies and then threading the device into the hole. The large hole can compromise the integrity of the vertebral bodies, and if drilled too posteriorly, can injure the spinal cord. Third, the end plates of the vertebral bodies are usually destroyed during the drilling. The end plates comprise very hard bone and help to give the vertebral bodies needed strength. With the end plates destroyed, the cylindrical device is now harder than the bone of the vertebral bodies and the vertebral bodies tend to collapse, "telescope," together. The telescoping causes the length of the vertebral column to shorten and can cause damage to the spinal cord and nerves that pass between the two adjacent vertebrae.
It is desirable to have a fixation device which not only eliminates the need to implant pedicle screws into the vertebrae but also which connects to the strong anterior vertebral bodies. The device should be easy to place and should prevent potentially damaging telescoping of adjacent vertebrae.